The present invention relates to a flexible packing laminate in the form of sheets or of a web intended for the manufacture of packing containers with good gas barrier characteristics. The laminate comprises a carrier a layer of dimensionally rigid, but foldable, material and a gas-tight metal foil laminated to one side of the carrier layer. The invention also relates to a method for the manufacture from the laminate. Moreover, the invention relates to the utilization of such a laminate for the manufacture of packing containers.
In the technology of packaging, the use of consumer packages of non-returnable character has been known for a long time for the packaging and handling of, among other things, liquid foodstuffs such as milk, juice etc. A very prevalent group of these so-called non-returnable packages is manufactured from a material having a carrier layer of paper or cardboard and outer and inner liquid-tight coatings of plastics, usually thermoplastics. In many cases, depending on the product which is to be packed, the material in these packages is also provided with further layers of other material, e.g. aluminium foil or plastic layers, in addition to those mentioned here.
The composition of the packing material is aimed at creating the best possible protection for the product which is to be packed, while also providing the packages with sufficient mechanical strength and dimensional stability so as to make possible a convenient handling. To bring about mechanical rigidity that provides mechanical protection for the product and makes the package dimensionally stable so that it can be readily transported and gripped by hand, the material in these packages is often provided with a relatively thick carried layer of paper or cardboard. Such a material, however, has no protection from liquids or gases and the good rigidity in the material is rapidly lost, if it is subjected to moisture or liquid which is absorbed into the material. Frequently, to impart liquid-tightness to the material both sides, the carrier layer are provided, with a plastic coating. If this coating is thermo-plastic, it can be used for sealing plastic layers to one another through so-called heat-sealing. In this manner, packing containers can be sealed and made to remain in their intended shape by heat-sealing thermoplastic-coated, overlapping material panels to one another in a tight and durable sealing joint.
Packing containers of the type mentioned here are manufactured either from blanks punched out in advance or from a web which has been prepared with appropriate decoration and with a crease line pattern facilitating the folding. Packing containers are manufactured from such a web by joining together the longitudinal edges of the web in an overlap joint so as to form a tube. The tube is subsequently filled with the intended contents and divided into closed container units through repeated transverse seals of the tube at right angles to the longitudinal axis of the tube. After suitable folding of the packing material into the tube, the material in the container units is converted to the desired geometrical shape. Generally, a parallelpided geometry is desired wherein the tube is provided with longitudinal folding lines and with double-walled, triangular folding lugs at the corner of the packing container.
During the forming of the packing container, the laminated packing material is subjected to stresses which become especially great during the folding course of the material. Due to the magnitude of the material thickness of the carrier layer, the folding the plastic coating on one side to be subjected to strong stretching while the plastic coating on the other side is compressed to a corresponding degree along the folding line. Because of the considerable extensibility of the plastic coatings, such a material folding only rarely causes the plastic coating to break or be damaged and lose its liquid-tightness. However, the situation is aggravated if the packing material also comprises an aluminum foil coating because aluminum foil has a low extensibility with respect to the plastic coatings and thus tends to crack during folding of the material.
Even though a single 180.degree. fold of the packing material normally will not have any serious consequences regarding liquid-tightness and gas-permeability, considerable difficulties arise, however, when the material is to be folded along two crossing crease lines (so-called crossings). This is often the case in external sealing regions which occur on this particular type of packing container. Generally, to form the seals, the plastic coatings facing towards the inside of the packing container are heated to a melting point along the edge zone which are to be sealed to one another. The heated plastic layers are then pressed against one another to form a sealing fin and are held together through material fusion on the outside of the packing container. Such a sealing fin comprises double material layers, and so as not to be an obstacle, the sealing fin often is folded down to lie flat against the outside of the packing container. Consequently one of the material layers of the sealing fin undergoes 180.degree. folding. The container wall in the region of the folded down sealing fin thus comprises three material layers, i.e., has a threefold material thickness. Such a sealing fin often runs along one or more of the lateral surfaces of the packing container that are subjected to a 180.degree. folding along a crease line at rignt angles to the sealing fin when forming parallelpipedic packages. Consequently, the material thickness in certain regions of the packing container will amount to six times the thickness of the laminate. During such a 180.degree. folding transversely to the sealing region, the outermost coated material layer will be subjected to very strong tensile stresses with subsequent extensions and crack formations in the material. These tensile forces often are so great that not only the aluminium foil included in the laminate, but also the thermoplastic coating crack, such that the packing container wall loses its liquid tightness characteristics in these particularly exposed regions of the packing container.